(a) Field of the Invention
The present invention relates to a rechargeable lithium battery and, more particularly, to a rechargeable lithium battery having good performance characteristics.
(b) Description of the Related Art
Generally, when transition metal oxides are employed for use in the positive electrode of rechargeable lithium batteries, carbon-based materials are among the best choice of active materials for the negative electrode. Owing to the use of the carbon-based active materials, the potential safety problem present in metallic lithium-based batteries can be prevented while achieving a relatively higher energy density as well as the reasonable shelf life.
The carbon-based materials can be largely classified into two categories of crystalline graphite and amorphous carbon. Crystalline graphite has a wider electrochemical voltage window and a higher energy density than amorphous carbon. For this reason, crystalline graphite is more frequently used for the negative electrode as the active material. Crystalline graphite having the desired specific properties such as density and interlayer spacing can be prepared by controlling the temperature of the heat treatment. Alternatively, doping with boron as catalyst may increase the capacity of coke type materials to a suitable degree. Several attempts are made to increase the catalyst content to be contained in the resulting graphite structure as it is conceived that the catalyst content acts as a critical factor for obtaining the desired crystalline structure.
The choice of electrolyte solvents is critical for the battery performance characteristics. When propylene carbonate is employed for the electrolyte solvent use, the graphite-based negative active material is liable to excessively react with propylene carbonate. Therefore, the available candidates of electrolyte solvents for ensuring a relatively stable reaction will be ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate. Among them, ethylene carbonate is usually recommended as the electrolyte solvent as it ensures a reasonable cycle life of the battery. In addition to the advantage, ethylene carbonate also has a disadvantage in that as its content increases, the freezing point of the electrolyte is elevated. This is due to the fact that ethylene carbonate has a relatively higher melting point exceeding the ambient temperature.
In order to solve such problems, it can be conceived that ethylene carbonate is replaced by propylene carbonate while preventing its excessive reaction with the graphite-based active material. This is performed by doping boron to the graphite-based active material. However, even in boron-doped active material, propylene carbonate may induce the aforementioned problem because the active material preparing process does not adequately performed. For example, in the active material preparing process, when boron is coated on the host material and heat-treated at high temperatures under an air atmosphere, boron reacts with nitrogen contained in air to thereby form boron nitride or is liable to be volatilized. In these cases, the desired properties of the resulting electrolyte can not be expected.
It is an object of the present invention to provide a rechargeable lithium battery which has good performance characteristics such as shelf life, capacity and charge and discharge efficiency.
It is another object of the present invention to provide a rechargeable lithium battery which can use propylene carbonate as an electrolyte solvent without any dropping of battery performance characteristics.
These and other objects may be achieved by a rechargeable lithium battery having a negative electrode with a graphite-based active material with boron as a donor and a positive electrode with a transition metal oxide-based active material. A separator is interposed between the negative and positive electrodes. The positive and negative electrodes and the separator are all saturated with an electrolyte. The electrolyte contains at least 51 percent by volume of cyclic carbonate and chain carbonate of 49 percent by volume.